mounting van seats on a battery box
Im working on my first conversion, and have a couple of seats out of a van (with integrated shoulder belts) that I want to bolt in. The risers under the seats are 9" tall on the short end, and in addition I have 2" of flooring and insulation. I am wondering about building a box that opens from the top that goes all the way to the original floor and using it both to mount the seats and store my batteries in the box. This would ideally include both the vehicle starting battery and a 200 amp hour LiPo accessory battery.
Assuming that I do the engineering and work with a professional welder and size the box material so it is strong enough to support the seats in a crash, I am wondering:
1) Is there anything inherently unsafe about having the batteries under the passenger seats?
2) will a inside height of 11" be enough room to fit industry-standard batteries?
3) Do I have to vent the battery box 'outside' or can it be vented into the interior of the bus?
4) Any other reasons this is a bad idea?